Machines used for machining plate and sheets have a fixed bottom tool or die and a top tool or punch moving reciprocally to and from the fixed tool. The movable tool can be driven with the aid of a simple crank movement. Such machines are punches and/or nibbling machines, for example. The invention is directed to this kind of plate or sheet working machines.
Between each working stroke of the punch the workpiece ("sheet") is moved a definite distance in a certain feed direction. Feeding the workpiece in this way can be done manually or with the help of purely mechanical means which are coordinated with the tool movement in some way. In certain cases, such as in single stroke punching, the top tool movement is arrested after each working stroke by disengaging the eccentric shaft and simultaneously braking it at the top end position of the crank movement.
In numerically controlled machines, feeding the workpiece is done with the help of servo devices which move the workpiece in one direction, or in two directions usually at right angles to each other. These servo devices can consist, for example, of d.c. or hydraulic motors which give the workpiece a definite feed travel during each feed step via a screw spindle. Electronic control equipment in the form of a computer is used to control the servo devices according to a desired program. Digital-analog control signals are sent by the computer to the servo devices according to a definite program.
During the part of the working stroke, where the punch is in contact with the workpiece or passing through it, the workpiece must be kept stationary and feed must therefore be blocked. Blocking must begin when there is a definite minimum distance between punch and die, and this distance should be somewhat greater than the thickness of the workpiece. The blocking function must be retained until the punch, after going through its bottom turning position, has once again departed a corresponding distance from the die. There is naturally no obstacle in already beginning or terminating the feed movement blocking operation at greater distances between the punch and die than what is unavoidably necessary, but to best utilize the available portion of the punching cycle time for feeding, and thereby to optimize the production capacity of the machine, it is desirable that blocking is started immediately before the tool reaches the workpiece and is terminated immediately after the tool leaves the workpiece. There is then the maximum time available for each feed step.
In a simple symmetrical crank movement, each angular position of the crankshaft corresponds to a definite distance between the outer turning position of the reciprocating piston and its actual position, i.e. the distance between punch and die in the case in question. The rotation of the crankshaft can thus be utilized to indicate the distance of the punch from the die. This condition can be utilized in numerically controlled machines for determining the instant when blocking the workpiece shall be started and terminated, in response to the thickness of the workpiece. To start with, a certain thickness of the workpiece is considered, and the distance between punch and die which is to be maintained for allowing the feed of the workpiece is determined. This distance is used to determine the corresponding angular position of the crankshaft for the initiation and termination of blocking. An indicator in the form of a cam plate which coacts with a contact device is attached to the crankshaft. The contact device gives a signal to the computer controlling feed that it is to perform the blocking operation during the period of the work cycle when the cam plate passes the contact device. The peripheral length of the cam plate coacting with the contact device can be adjusted to the plate thickness in question by suitable means.
In a further case in the prior art, electrical contact between the punch and the upper surface of the workpiece is utilized to determine the time at which the feed movement of the workpiece shall be blocked, or initiated once again.
These known devices lead to relatively complicated structures, however, especially when they include means which are fitted in the immediate vicinity of the area worked, and are thus a hindrance, e.g. in cases where the machine has apparatus for automatic tool changing or other auxiliary apparatus which must be given a place in the immediate vicinity of the work area.